1. Technical Field
The invention relates to a pigment dispersion and an ink composition using the pigment dispersion.
2. Description of the Related Art
When pigment is used as a colorant in colored compositions of various kinds, it is important to secure dispersibility and dispersion stability of the pigment in the form of a solid. By using a pigment dispersion that exhibits excellent dispersibility and dispersion stability of the pigment, it is possible to obtain a curable composition or an ink composition that exhibits a uniform color hue. In particular, a pigment that exhibits excellent light fastness is generally included in the ink composition, and when this pigment has a problem in its dispersibility, it may cause various problems such as nonuniform color tone, or degradation in dischargeability when the ink composition is used for an ink jet recording method.
In recent years, an ink jet recording method has been attracting attention as an image recording method in which an image is formed on a recording medium based on image data signals. The ink jet recording method also has advantages in that the recording device produces less noise, and that an image with high sharpness can be recorded at low running cost by discharging extremely small droplets.
According to the inkjet recording method, printing can be performed not only on ordinary paper but also on recording media that do not absorb a liquid, such as a plastic sheet or a metal plate. Further, in order to realize a higher printing speed and a higher image quality, there is demand for shortening of time for drying and curing the ink. In this regard, there is a recording method in which ink that can be cured in a short period of time upon irradiation with actinic energy rays is used as an ink for inkjet recording. According to this method, a sharp image can be formed by curing ink droplets by irradiating with actinic energy rays immediately after the printing. The curable ink composition for inkjet of this kind needs to have a high degree of pigment dispersibility and stability over time thereof, in order to form a highly sharp image in which the ink exhibits an excellent color forming property, and to discharge the ink composition in a stable manner.
In order to impart a vivid color tone and a high coloring power to the ink composition, microparticulation of the pigment is necessary. In particular, in the case of ink composition that is used for inkjet recording, discharged ink droplets largely affect the sharpness of the image. Therefore, it is necessary to discharge the droplets in small amounts and use particles having a smaller size than the thickness of the cured film formed from the ink composition. However, as the pigment particles are micronized in order to obtain a high coloring power, it becomes difficult to disperse the particles and an aggregation tends to be formed. Moreover, there may be a problem in that the viscosity is increased as a result of excessive addition of the dispersant. Since the formation of a pigment aggregation and the increase in viscosity of the ink composition both adversely affect the dischargeability of the ink, it is not preferred to use an ink composition in which an aggregation of pigment is formed or the viscosity is increased for inkjet recording.
Further, when an ink composition is used for inkjet recording, the ink composition must have an excellent heat cycle property. The ink composition for inkjet recording is contained in a cartridge and is heated in order to lower the liquid viscosity at the time of discharging, but the temperature of the ink composition is decreased when discharge is not performed or during storage. Therefore, the ink composition is subject to temperature changes of repeating heating and cooling. These temperature changes also adversely affect the pigment dispersibility, and there are problems in that the dispersibility of the pigment is lowered through time, and chances of forming an aggregation of the pigment or increasing the viscosity are increased.
Various proposals have been made on a dispersant used for obtaining a stable ink composition, including an ink composition in which a pigment derivative is used as a dispersant in order to improve the affinity for the pigment (for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-119414 and JP-A No. 2004-18656); an ink composition in which a polymer having a basic group is used as a dispersant with respect to a pigment of a specific kind such as phthalocyanine or quinacridone (for example, JP-A No. 2003-321628); an ink composition in which a dispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and a monomer of a specific kind that dissolves the dispersant, which ink composition does not include an organic solvent (for example, JP-A No. 2004-131589); and an ink composition in which a graft copolymer having a hetero ring residue that forms an organic pigment is used as a dispersant (for example, JP-A No. 2007-9117).
It is true that the pigment dispersant or the ink composition disclosed in these documents can finely disperse the pigment, and that the stability of the ink composition can be increased as compared with conventional ink compositions. However, stability of the ink composition, particularly dispersion stability of the ink composition after a long-term storage or after undergoing repeated temperature changes, has yet to be improved.
In view of the above, a block pigment dispersant is proposed in which a block polymer including blocks each supporting different properties such as solvent solubility is used (for example, Examined Japanese Patent Application Publication No. 3590382, JP-A No. 2007-112932 and JP-A No. 2008-56806). Examples of the block polymer include a block polymer including a methacrylate block, a block having an amino group and a block having hydroxyacrylate, a polymer having a urethane block and an acrylate block, and a block polymer formed by including plural blocks each consisting only of an acidic group. In any of these cases, further improvement has been desired from the viewpoint of achieving dispersion stability that is sufficient to be applied to an ink composition containing micronized particles of the pigment.